let’s say I want to create a custom module and sell it with a monthly subscription fee.
How would be possible to protect/hide the code when the plugin is installed, in order to avoid modifications and manipulations after the installation?
If you check the SuiteCRM Store, you can register there as a software publisher, and they will provide all that service (for a fee). It is probably worth it. I don’t see people selling add-ons outside the Store, the Store makes it really convenient and safer.